In a mobile communication system utilizing time division duplexing (TDD) of uplink and downlink signals, base stations (BSs) are required to be mutually synchronized in signal transmission in order to avoid generation of unwanted interference among different signals. One method to achieve synchronization of the BSs over the mobile communication system is to allow a first BS to sniff a radio signal from a second BS so that the first BS is time-synchronized to the second BS based on the radio signal. Furthermore, re-synchronization is sometimes required. To illustrate the need for re-synchronization, consider a situation shown in FIG. 1. A sniffing BS 110, originally time-aligned with an original master BS 120, loses synchronization with this BS 120 because, for example, the original master BS 120 becomes out-of-service. The sniffing BS 110 is then required to synchronize itself with a new master BS 130. It is possible that there was already an inherent timing difference between the original master BS 120 and the new master BS 130 in signal transmission. Besides, a signal path 125 between the original master BS 120 and the sniffing BS 110 is often different in length with another signal path 135 for signal transmission from the new master BS 130 to the sniffing BS 110. Hence, it is often that at the sniffing BS 110, a received signal sent from the new master BS 130 is not time-aligned with another received signal originated from the original master BS 120. Timing synchronization of the new master BS 130's signal at the sniffing BS 110 is required.
Due to various advantages, most of present-day and future mobile communication systems, such as a Long Term Evolution (LTE) system, use OFDM for data transmission. A correlation method for establishing timing synchronization of an OFDM signal is provided by Jan-Jaap van de Beek et al., in “Low-Complexity Frame Synchronization in OFDM Systems,” Proceedings of IEEE International Conference on Universal Personal Communications, 1995, the disclosure of which is incorporated by reference herein. However, this correlation method requires a large observation interval which is not practical for a sniffing BS.
Most mobile communication systems embed pilot symbols in OFDM signals. Timing synchronization may be achieved by further utilizing these pilot symbols. For illustration, FIG. 2 depicts a time-frequency plane on which pilot symbols are located, where the pilot symbols are arranged according to a LTE specification as an illustrative example. Consider a time instant 241. There are two pilot symbols 210, 220 separated by a frequency spacing 230. The presence of a timing offset is translated into a proportional phase shift for each of the two pilot symbols 210, 220. It follows that the timing offset can be estimated based on an observed phase shift between the two pilot symbols 210, 220. Since the observed phase shift has a 2π ambiguity, there is a maximum detection range of the timing offset that can be estimated. In many practical situations, the actual timing offsets often exceed this maximum detection range. US2014/0036779A1 and US7558245B2 provide timing-offset estimation methods that overcome this limitation, but these methods involve high implementation complexity.
There is a need in the art to have a timing-synchronization technique that utilizes pilot symbols to estimate a timing offset greater than the aforementioned maximum detection range. The technique is not only applicable for mobile communication systems but also useful for other wireless communication systems, such as a wireless local area network (WLAN) having multiple access points for coordinated transmission.